Triarylamines have been discovered to be excellent charge transport compounds in electrophotographic photoconductors. In electrophotography, a latent image is created on the surface of an imaging member which is a photoconducting material by first uniformly charging the surface and selectively exposing areas of the surface to light. The difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner.
Processes for the preparation of certain charge transport compounds are known reference for example, U.S. Pat. Nos. 4,299,983; 4,485,260; 4,240,987; 4,764,625; and 4,299,983, the disclosures of each of these patents are incorporated herein by reference in their entirety. These and other references illustrate the Ullmann condensation of a diarylamine and a haloaromatic compound at high temperatures.
The most common synthesis of triarylamines involves the coupling of a diarylamine and a haloaromatic compound, preferably an iodoaromatic compound, in the presence of a base and copper at high temperature (around 200.degree. C.) as was first reported by F. Ullmann in 1903. These reactions are typically conducted in high boiling solvents such as nitrobenzene or o-dichlorobenzene. In addition, the reaction times are typically quite long. The drawbacks of the high reaction temperature and long reaction time limit the application of this reaction for the efficient preparation of triarylamines for use as hole transport molecules in photoconductors.
Another process for tertiary amine preparation is disclosed in the Turner et al U.S. Pat. No. 4,764,625. Turner et al disclose that tertiary amines can be prepared by the condensation of a secondary amine with a mono- or diiodoaryl compound in the presence of potassium hydroxide and copper and a solvent mixture of C.sub.13 -C.sub.15 aliphatic hydrocarbons having a boiling point of at least 170.degree. C. However, in order to complete the reaction disclosed by Turner et al, a large excess of secondary amine is needed because of the limited solubility of the product in the solvent. In addition, increased difficulty is encountered in separating any inorganic solids from the desired reaction product, resulting in a reduced yield of the triarylamine product.
Various copper catalysts have been disclosed for use in the condensation reaction. A process is disclosed in DE 4,427,121 in which a diarylamine is condensed with an iodoaryl compound in the presence of Cu (OAc).sub.2 and Zn. N,N'-diphenyl-N,N'-di(3-tolyl)-p-benzidine (TPD) was generated in a yield of 71% after the reaction was heated at temperatures of 220-230.degree. C. for four hours in a large excess of 3-methyldiphenylamine.
Recently, the Goodbrand et al U.S. Pat. Nos. 5,654,482 and 5,648,539 disclosed a process in which aniline is reacted with iodoaryl compounds in the presence of an alkali metal hydroxide and a ligated copper catalyst (for example, 1,10-phenanthrolato-copper (I) chloride).
However, there remains a need for an efficient process for the preparation of triarylamines at high yield under relatively mild conditions. In addition, there remains a need for a simple purification process to obtain high purity triarylamines, particularly in good yields.